Selective hydrogenations of unsaturated hydrocarbon compounds are of high industrial significance. The pyrolysis of naphtha for the production of ethene, propene, butanes, 1,3-butadiene and aromatics is a key process in the modern petrochemical industry. For the nearly complete removal of alkynic compounds from the C2, C3 and C4 cuts, selective hydrogenations are generally used.
For instance, the hydrogenation of acetylene is an important industrial process to remove traces of acetylene in the ethylene feed for the production of polyethylene. Because acetylene poisons the catalyst for the polymerisation of ethylene to polyethylene, the acetylene content in the ethylene feed has to be reduced to the low ppm range. Moreover, economic efficiency requires high selectivity of acetylene hydrogenation in the presence of an excess of ethylene to prevent the hydrogenation of ethylene to ethane.
Typical hydrogenation catalysts contain palladium dispersed on metal oxides. While palladium metal exhibits high activity, e.g., in the hydrogenation of acetylene, it possesses only limited selectivity because of the formation of ethane by total hydrogenation and C4 and higher hydrocarbons by oligomerisation reactions.
Modification of palladium catalysts by adding promoters or alloying with other metals has been shown to result in an increased selectivity and long-term stability in acetylene hydrogenation. For instance, increased selectivity in semi-hydrogenation of alkynes was reported for Pd combined with Ag (U.S. Pat. No. 4,404,124; and D. C. Huang et al., Catal. Lett. 53, 155-159 (1998)), Sn (S. Verdier et al., J. Catal., 218, 288-295 (2003)), Au (T. V. Choudhary et al., Catal. Lett., 86, 1-8 (2003), Ni (P. Miegge et al., J. Catal., 149, 404-413 (1994), and Pb (W. Palczewska et al., J. Mol. Catal., 25, 307-316 (1984). However, the catalytic performance of these modified Pd catalysts remains insufficient and further improvements in selectivity may decrease the costs for the production of polyethylene. In addition to selectivity, the long-term stability of palladium catalysts has to be improved further.
The C3 cut (propylene) is generally purified by selective hydrogenation of propyne (methylacetylene) and propadiene (allene), and the obtained propylene may be further processed to polypropylene.
Another important selective hydrogenation in industry is the removal of traces of 1,3-butadiene from the C4 fraction after the extractive separation thereof. Pd/Al2O3 catalysts are commonly used in this reaction. Furthermore, the selective hydrogenation of 1,5-cyclooctadiene, obtained by cyclic dimerization of 1,3-butadiene, to cyclooctene on Pd/Al2O3 and of benzene to cyclohexene on ruthenium catalysts are of importance.
In all of these selective hydrogenations, further improvements of the selectivity to the desired product and an increased long term stability of the used catalyst have been strongly desired.
The intermetallic compounds PdGa or Pd3Ga7 are described by E. Hellner et al. in Z. Naturforsch. 2a, 177-183 (1947) and by K. Khalaff et al. in J. Less-Common Met. 37, 129-140 (1974). However, any catalytic potential of these compounds has been unknown so far.
The use of ordered intermetallic compounds as catalysts in a variety of different reactions is generally described in US 2004/0126267 A1 and WO 2004/012290 A2. However, these documents fail to disclose the application of this type of compounds to hydrogenations, let alone selective hydrogenations. In fact, the focus of these references is on their use in fuel cells. In addition, these patent applications fail to disclose the specific Pd based intermetallic compounds of the present invention.
It is therefore an object of the present invention to provide a process for the hydrogenation of unsaturated hydrocarbon compounds, in particular of ethyne (acetylene) in admixture with a large excess of ethene (ethylene) to afford ethene, which overcomes the drawbacks of the prior art as outlined above, and which shows an improved selectivity to the target product. It is another object to provide novel catalysts having the above beneficial properties in selective hydrogenation reactions, in particular in the selective hydrogenation of acetylene to afford ethylene when ethylene is present in an excessive amount.